1. Field of the Invention
The present invention relates to new and useful improvements in manufactured paper and, more directly, relates to the provision of resin absorbent papers for the manufacture of high and low pressure fiber reinforced plastic laminates with improved postforming characteristics.
2. Description of the Prior Art
Postforming laminates are laminates that, after initial curing in a press, can be reheated along an axis and bent to give a material that has smooth, round, undamaged surfaces in either the forward or the backward direction. Such bends are depicted in FIG. 1. Forward bend 1 (sometimes called a "bullnose bend"), where the decorative side (or layer) 3 of the laminate is in tension, is generally a large radius bend exhibiting a radius from 1/4 inch to 1/2 inch. Tight bends with small radii may lead to damage in the laminate that can appear upon forming or appear at a later time in use and give failures that would be correctable only at great expense. Attempts to bend the decorative side in tension around a very tight radius (e.g., 1/16 inch to 3/16 inch) generally are unsuccessful due to the relatively high modulus with low elongation at break of the decorative layer. In general, forward bends of laminates are done at larger radii.
In contrast, a backward bend 2 of a laminate, where the decorative side 3 of the laminate is in compression and the core side 4 (shown in FIGS. 2 and 3 to be comprised of internal core sheets 5 covered with one or more postforming sheets 6 or 7, respectively) is in tension, generally is postformed to much tighter radii. This is due to the fact that the core side 4 of the laminate, which is impregnated with phenol/formaldehyde resin, can, by using appropriate paper or resin modifications, be made to have a lower modulus with a higher elongation at break than the decorative side 3. Another reason for the tighter attainable radii is the fact that the outer (tension) layer of the core side 4 can be modified to give a laminate that is better able to tolerate tensile strains imposed on it during bending. Such modifications of the decorative side 3 are not possible due to appearance and physical property requirements. Summarizing, decorative postforming laminates must be able to be bent with either the decorative or the core side in both tension and compression without affecting the appearance of the decorative surface.
A number of approaches have been taken to improve the performance of decorative postforming laminate bends. The prior art comprises various modifications to improve the bending ability of the decorative laminates in the forward direction. Improvements in the forward bends can be made by changing the furnish of the paper. For example, U.S. Pat. No. 3,327,084 teaches the use of low coarseness (thin and long) fibers as being superior to the use of thicker, high-coarseness fibers. Improvements in crazing, which is the undesirable tendency of the laminate to have cracking on the surface, were noted with the lower coarseness fibers. Specifically, high coarseness fibers such as southern loblolly pine were listed as being poor for postforming bends. In addition, the use of low denier synthetic fibers such as nylon and acrylic fibers was found to give improvements in crazing when used at levels of between 35% and 100% in the paper. Although this technology appears viable, the use of low coarseness natural fibers generally limits the papermaker to juvenile trees or slow-growing northern species, and synthetic fibers are considerably more expensive than wood fibers.
A second means of adding postformability to a laminate, especially for bending in the forward (or, bullnose) direction, is to modify the melamine resin that is used to impregnate the decorative layer. A variety of modifications to the decorative layer have been described including the use of caprolactams (U.S. Pat. No. 2,584,177), glucosides (U.S. Pat. No. 2,773,788), carbamates (U.S. Pat. No. 2,937,966), mannatin (U.S. Pat. No. 3,194,723), epoxies (U.S. Pat. No. 4,046,937), and polyethyleneglycols (U.S. Pat. No. 4,405,690).
Several prior art references describe modifications to the phenolic resin in the core sheets to improve post-forming characteristics. These modifications include the use of water/glycerol, as in Canadian Patera No. 778,750, to enhance the performance of forward bends by enabling the core to undergo greater compressive deformation, inducing less tensile strain in the decorative layer. In Canadian Patent No. 894,859, the use of glycols and fatty acid oils as coreactants for the phenolic resin is described. In all cases, however, the use of these materials was to improve the appearance of the forward bend.
Current commercial postforming laminates employ one or several lightweight sheets on the back of the laminate. This laminate construction is shown in FIG. 2 to be comprised of the decorative layer (or side) 3 and the core side 4. The lightweight postforming sheets 6 that are used on the back of this laminate are a multi-walled sack paper that generally is made from northern low coarseness softwood. That pulp is understood to be subjected to mild (low energy) high-consistency treatment (e.g. Frotapulper from Ingersoll-Rand) and, the web is dried in an unrestrained manner (Flakt drying). This Flakt drying process, which also allows free shrinkage in the cross-direction; gives high (6-7%) cross-directional (CD) stretch. This cross-directional stretch imparts the desirable bending characteristics to the laminate which is heated along a machine-direction (MD) axis and is bent in the cross-direction. It has been accepted that high CD stretch in a paper sheet is necessary for use in the postforming layer of a laminate.
Paper of this type is available in low basis weights of 75 lbs./3000 ft.sup.2 from RePap Industries in LaPas, Alberta, Canada. The primary market for this paper is bag stock rather than laminates. Although this paper is known to give good bending characteristics to the laminate, it suffers from several deficiencies, including inherently poor treatability, due to sheet construction, low basis weight, and relatively high cost, due to air-drying techniques needed to induce CD stretch. For standard horizontal grade postforming laminates of about 0.037 inch thickness, the postforming sheets comprise about 1/3 of the mass of the core of a laminate, and two 75 lbs./3000 ft.sup.2 sheets must be used. Therefore, twice as many linear feet of paper must be treated, as compared to having a product which requires only one sheet of about 150 lbs./3000 ft.sup.2. In addition, since the lighter weight papers are designed as extensible sheets, they tend to give poor dimensional stability to laminates. The use of multiple sheets with low basis-weight also requires additional collating costs in the layup of the laminate. Thus, the current commercial product results in relatively slow production rates and higher costs to the laminator.
An object of this invention, therefore, is to provide a sheet that overcomes many of the problems associated with highly extensible, light weight commercial products that currently are being employed. Specifically, it is an object of this invention to provide a high basis weight sheet that does not require high CD stretch and is easier to treat and less costly to produce and use than the lighter-weight products, while still providing excellent surfaces in both forward and backward bends.